This invention relates to a field emission cold cathode element having at least one minute emitter electrode with a sharp-pointed tip which is close to a gate electrode and from which electrons are emitted.
It is known, as described in Journal of Applied Physics, Vol. 47, No. 12 (1976), pp. 5248-5263, to produce a field emission cold cathode element by forming an array or arrays of a number of microscopically minute and conical emitter electrodes on a conducting substrate. The cold cathode element is fabricated by forming a dielectric layer on the conducting substrate, overlaying the dielectric layer with an electrode layer, for each emitter electrode forming a hole in the electrode layer, through that hole etching the dielectric layer to expose the substrate surface beneath the hole and growing a conical emitter electrode on the exposed substrate surface by a physical vapor deposition method until the tip of the conical emitter electrode nears or protrudes into the hole in the electrode layer. The electrode layer on the dielectric layer becomes a gate electrode for drawing the current emitted from every emitter electrode and controlling the emission current. Usually a voltage of 100-300 V is applied between the gate electrode and the substrate to which the emitter electrodes make electrical connection.
In this cathode element the conical emitter electrodes are about 1 .mu.m in height (the dielectric layer is about 1 .mu.m in thickness), and the hole in the gate electrode layer for each emitter is about 1 .mu.m in diameter. Since the sharp-pointed tip of each emitter electrode is so close to the gate electrode a strong electric field acts at the emitter electrode tip, and electrons are emitted from the emitter tip when the field intensity reaches 2 to 5.times.10.sup.7 V/cm. A large number of identical emitter electrodes are arranged on the substrate in closely packed arrays to provide a planar cold cathode element that can emit a large current. Compared with conventional hot cathode elements, this cold cathode element has advantages such as higher current densities and less fluctuations of the velocity of emitted electrons. Furthermore, by comparison with conventional field emission cathode elements having a single, relatively large emitter electrode, this cathode element has advantages such as reduced current noises, lower gate voltages for useful emission and operability in lower vacuums.
With respect to each conical emitter electrode in the above described cold cathode element the emission current depends greatly on the position of the emitter electrode tip relative to the gate electrode and, hence, on the height of the emitter electrode. In the cathode element having a large number of conical emitter electrodes, some dispersion of emitter electrode heights is inevitable, and hence there are some variations in the emission characteristics of the individual emitter electrodes. When the variations are considerable, the maximum emission current of the cathode element must be reduced since the maximum emission current is restricted by the allowable maximum emission characteristic of one emitter electrode which makes the highest emission at a given voltage. An already known measure for reducing variations in the emission currents is to make the gate electrode layer relatively thick such that the position of the tip of each emitter electrode becomes above the middle plane of the gate electrode layer.
However, another problem is augmented by thickening the gate electrode layer. The problem arises from temperature changes which the cathode element experiences during the manufacturing process. In confining the cathode element in a vacuum enclosure, it is necessary to discharge gases that are adsorbed by the cathode element and other components in the vacuum enclosure in order that the cathode element can be long operated in high vacuum. Usually the gases are extracted while the interior of the enclosure is maintained at a temperature above 500.degree. C. The heating to such a high temperature and subsequent cooling induce interlayer stresses between the gate electrode layer and the underlying dielectric layer since the two layers differ in thermal expansion coefficients, and the stresses increase as the gate electrode layer becomes thicker.
From another aspect, for extending the life of the above described cathode element and enhancing the reliability of same, it is desirable that the material of the gate electrode layer has a high melting point and is refractory because there are possibilities of collisions of a portion of electrons emitted from the emitter electrodes or electrons reflected from other electrodes in the vacuum enclosure against the gate electrode and occurrence of micro-discharges between the gate electrode and the emitter electrodes. However, conducting and desirably refractory materials are generally greatly different in thermal expansion coefficients from silicon dioxide which is usually used for the dielectric layer under the gate electrode layer. Therefore, the aforementioned stresses further increase when the gate electrode layer is formed of a refractory material and made sufficiently thick.
With respect to the gate electrode in field emission cold cathode elements of the above described type, there are several proposals.
JP 4-167324 A proposes a two-layer structure of the gate electrode, consisting of a first gate layer which is a polycrystalline silicon layer formed directly on the dielectric layer and a second gate layer which is a metal silicide layer formed on the polycyrstalline silicon layer. The second gate layer of a metal silicide, which is very high in melting point, is employed with the intention of preventing lowering of the resistivity of the gate electrode by oxidation and deformation of the gate electrode in the vicinity of each emitter electrode. The metal silicide layer is underlaid with the polycrystalline silicon layer to ensure good adhesion of the gate electrode to the dielectric layer. However, if this two-layer gate electrode is made sufficiently thick significant stresses will be induced by different thermal expansions between the gate electrode and the dielectric layer and also between the first and second gate layers.
JP 4-284325 A also proposes a two-layer structure consisting of a usual gate electrode layer and an upper, protective layer formed of a conducting material excellent in corrosion resistance. This reference shows a three-layer structure produced by inserting a thin layer between the above two-layer gate electrode and the dielectric layer in order to improve adhesion. However, such multilayering leads to increased interlayer stresses.
JP 57-187849 A shows forming a small, annular gate electrode for each of a number of conical emitter electrodes to thereby control the emission currents of the emitter electrodes individually. Since the dielectric layer below the gate electrodes is formed over the entire area of the substrate (though it is removed in narrow circular regions where the respective emitter electrodes are formed), the formation of the small annular gate electrodes results in that the dielectric layer is exposed over the major area. Therefore, in operation of the cathode element in a vacuum, it is likely that the deposition of electrons and ions on the dielectric layer causes changes in the potential at the plane of the dielectric layer surface and resultant variations in the trajectories of electron beams emitted from the emitter electrodes.